Measuring Instrument

ABSTRACT

A measuring instrument ( 1 ) having at least one level sensor ( 3 ) for detecting a level of a medium, which outputs a level signal; wherein the measuring instrument ( 1 ) has at least one pressure sensor ( 5 ) from which a pressure measurement signal can be read at the output; wherein the level sensor ( 3 ) and the pressure sensor ( 5 ) have a shared process connection ( 7 ).

The present invention relates to a measuring instrument with the features of the generic part of claim 1.

Measuring instruments with at least one level sensor for detecting a level of a medium, which outputs a level signal, are known from the prior art. Such measuring instruments frequently comprise level sensors, which are designed as vibration level sensors. Such vibration level sensors comprise a diaphragm that can be excited to vibrate via a drive, by means of which diaphragm a mechanical oscillator disposed on the diaphragm can be excited to vibrate. Depending on a coverage status of the mechanical oscillator with a feed material and depending on the viscosity of this feed material, said oscillator vibrates with a characteristic frequency and aplitude, wherein the frequency and/or the amplitude is monitored and changes are detected by the vibration sensor and can be converted into a measuring signal.

Measuring instruments with pressure sensors are moreover known from the prior art, wherein capacitive [SJM1] [BF2J]¹ pressure measurement cells are utilized as pressure sensor. Such capacitive pressure measurement cells detect an ambient pressure by means of a measuring diaphragm disposed on a measuring cell body, wherein electrodes are disposed both on the measuring cell body as well as on the measuring diaphragm. Depending on the pressurization, the deflection of the measuring diaphragm changes and therefore a spacing of the electrodes, so that a change in capacity is detected by the measuring capacity are formed by the electrodes, making it possible to deduce the applied measuring pressure therefrom.

During their deployment, the individual measuring instruments are connected with a measuring bypass' by means of a so-called process connection. In particular during applications in the food industry or in the pharmaceutical industry, it is necessary to warrant certain hygiene standards and in particular cleanability of the measuring instruments used.

With the aforementioned applications it is perceived as disadvantageous that respective hygiene standards with respect to cleanability and sealing must be complied with for each individual process connection, which respectively results in increased complexity during the manufacture of the respective process connection.

The object of the present invention is to refine a measuring instrument according to the prior art such that these disadvantages are remedied.

A measuring instrument according to the invention with at least one level sensor for detecting a level of a medium, which sensor outputs a level signal, is characterized in that the measuring instrument comprises at least one pressure sensor, on the output side of which a measuring signal can be read, wherein the level sensor and the pressure sensor comprise a shared process connection.

In the present application, process connection is understood to be the mechanical interface of a measuring instrument by means of which the measuring instrument is connected to the environment.

By integrating a level sensor and a pressure sensor into a single process connection, a significant reduction in the manufacturing expense of this process connection is accomplished in terms of sealing, cleanability and hygiene standards, so that distinctly more favorable measuring instruments can be provided overall. Moreover, a distinctly more compact structural design of the measuring instrument is possible, so that the field of applications for the measuring instrument can also be augmented considerably.

The level sensor is preferably designed as vibration level sensor in a refinement of the invention. By using vibration level sensors, it is possible to resort to a well-proven technology with respect to the measuring results, so that high-quality sensors are available for level detecting.

For measuring the pressure, the pressure sensor is preferably designed as ceramic pressure sensor, which can be developed as capacitive pressure sensor, for example, or as metallic pressure sensor which can function based upon strain gauges, for example. Also in this instance this involves a known and well-proven pressure measurement technology, which supplies high-quality measuring results and for which suitable sensors are available for a plurality of measuring environments.

The process connection can be developed as flange or as a screw thread, for example, wherein the pressure sensor is disposed laterally to the level sensor and is axially aligned. In the present application, axial direction is understood to be a direction perpendicular to a diaphragm level of the level sensor.

By means of a pressure sensor configuration laterally to the level sensor, for example in a flange or in a screw-in thread, a front face of the process connection that is plane to the extent possible can be provided and a flush-mounted or at least almost flush-mounted connection of the measuring instrument or the sensors to the process environment can be accomplished. This is a particular advantage, in particular in view of the previously addressed topic of cleanability.

In one alternative embodiment, in which the process connection is designed as a screw-in thread and has an extension piece, the pressure sensor is disposed laterally to the level sensor and is aligned radially and is preferably disposed on the extension piece. By a lateral configuration of the pressure sensor relative to the level sensor and a radial alignment, i.e. an alignment perpendicular to the axial direction, this permits a more compact structural design, however by having to contend with extending the process connection in the axial direction.

In this case, the pressure sensor is aligned radially and is connected to the process environment by means of a sidewall of the extension piece. Such a form of installation can be advantageous, if only a limited installation surface is available for the process connection.

In addition, the measuring instrument can have a temperature sensor on the output side of which a measuring signal can be read.

The level signal and the pressure signal are advantageously fed into a shared evaluation electronics. Having a shared evaluation electronics will save further components in the structural design of the measuring instrument, so that by using only one evaluation electronics, manufacturing costs can be saved overall.

The evaluation electronics advantageously comprises a compensation module that is suitably designed to perform pressure compensation of the level signal.

In principle, the vibration frequency and vibration amplitude, in particular the resonance frequency and the associated amplitude of a mechanical oscillator of a level sensor vary depending on pressure. With the previous traditional application scenarios for level sensors, for example in bulk materials or liquids, these changes because of a pressure change do not have to be compensated, since as a result of a coverage of the mechanical oscillator with the respective bulk material, sufficiently large frequency and/or amplitude changes are caused, so that a switch state can be detected with certainty.

In more recent application scenarios, in particular during level monitoring of liquid gases, the change of the resonance frequency and/or amplitude of the level sensor caused by coverage with the bulk material is in the order of magnitude based upon a potential pressure changes solely due to a change of the resonance frequency and/or amplitude, so that erroneous detections of the level sensor are possible solely because of a pressure change. By combining the level sensor with a pressure sensor and the processing of both sensor signals in a shared evaluation electronics, the environmental pressure can be taken into account during the evaluation of the level signal and thus a compensation of changes in frequency and amplitude can be implemented based on pressure changes. In this manner, the application of level sensors for these new fields of applications is uniquely enabled.

For this purpose, the level sensor is ideally designed as vibration level sensor and the pressure compensation compensates a pressure-induced change of a resonance frequency and/or resonance aplitude of the mechanical oscillator of the vibration level sensor.

In a refinement, the temperature signal is also fed to the shared evaluation electronics, wherein the compensation module is preferably suitably designed to perform a temperature compensation of the level signal or of the resonance frequency and/or amplitude.

The present measuring instrument can be designed as flow limit switch or as pump protection circuit.

The measuring instrument is preferably designed according to the EHEDG TYPE EL Class I standard.

As a result of a measuring instrument according to the invention, compact pump protection can be accomplished which can be integrated into the pump in particular in confined spatial conditions. The pump is thus protected by a sole sensor signal both against running dry as well as against excessive high pressure and excessive low pressure conditions.

In some cases, the pressure sensor can be affected by the vibrations of the level sensor to the extent that an erroneous pressure measurement signal results. To prevent such interference of the pressure measurement signal, it is advantageous if the pressure sensor and the level sensor are not operated at the same time, at least temporarily.

According to the invention, a measuring instrument with at least one level sensor for detecting a level of a medium, which level sensor outputs a level signal, and at least one pressure sensor, on the output side of which a measuring signal can be read, wherein the level sensor and the pressure sensor comprises a shared process connection are therefore operated such that the level signal and the pressure signal are detected and evaluated, wherein the pressure sensor and the level sensor are operated at different times, at least temporarily.

For example, either both sensors can preferably be operated cyclically alternate, or the vibration sensor is switched on and switched off cyclically, wherein it preferably provides a trigger signal during the switched off phase, which reports the pressure measurement signal present at the same time as an invalid measuring value.

Preferably, a temperature signal is detected in addition to the level signal and the pressure measurement signal. Preferably, in addition a temperature compensation of the level signal or of the detected resonance frequency and/or amplitude is carried out.

In the following, the present invention is explained in detail with reference to the attached figures.

The drawings show:

FIG. 1 is an embodiment of a measuring instrument according to the invention with a flanged process connection,

FIG. 2 is a first embodiment with a threaded process connection,

FIG. 3 is a second embodiment with a threaded process connection,

FIG. 4a is a dependency of the resonance frequency of a vibration level sensor on an environmental pressure, and

FIG. 4b is a dependency of the resonance frequency of a vibration level sensor on an environmental temperature.

FIG. 1 illustrates a schematic representation of a measuring instrument 1 with a process connection 7 developed as flange 71.

In the center of the flange 71 that is preferably developed circular, a level sensor 3 is disposed, which in the present form is designed as vibration level sensor with a mechanical oscillator 33 and a drive 31 disposed on the rear. On the drive 31, which is developed for simultaneously detecting a resonance frequency f_(res) of the mechanical oscillator 33, a level signal can be read and fed into an evaluation electronics 9 via a first connecting line 35. In the present embodiment, the level sensor 3 is aligned in axial direction A, i.e. in that the drive 31 and the mechanical oscillator 33 essentially extend in axial direction A. In the embodiment illustrated in FIG. 1 a pressure sensor 5 of the present piezoresistive pressure measurement cell is disposed laterally to the level sensor 3, which pressure measurement cell operates based upon strain gauges and is produced by thin-film technology. On the output side of the pressure sensor 5, a pressure signal can be detected, which is also fed to the evaluation electronics 9 via a second connecting line 55.

The pressure sensor 5 furthermore comprises an integrated temperature sensor 6, on the output side of which a measuring signal can be read. The temperature measuring signal is likewise fed it to the shared evaluation electronics 9, where it is processed.

The evaluation electronics 9 comprises a compensation module 91, to which both the level signal as well as the pressure signal and the temperature signal is fed. In the compensation module 91, a pressure and temperature compensation of the level signal, i.e. a compensation of a pressure and temperature induced change of the resonance frequency f_(res) and/or amplitude of the mechanical oscillator of the level sensor 3 is performed, so that a significantly increased measuring accuracy of the level sensor 3 can be accomplished.

This is in particular necessary with measuring instruments 1, which are to be utilized in a large pressure and/or temperature range, for example during the level detection in liquid gas tanks.

The background for this is the following:

Gases, such as air or liquid gas, for example, are compressible. As the pressure of the gas increases, the gas becomes denser, i.e. there are more molecules within a specific volume. This impacts the vibration limit switch. With increasing pressure of the surrounding gas, the resonance frequency f_(res) or the amplitude of the mechanical oscillator 33 is slightly reduced by an increased resistance, which the compressed gas exerts on the mechanical oscillator 33.

If the mechanical oscillator for example operates within a wide pressure range, for example between −1 bar to 160 bar, then by varying the environmental pressure, the resonance frequency f_(res) and/or amplitude of the mechanical oscillator can be varied so greatly that this change is misinterpreted by the evaluation electronics 9 of a traditional level sensor 3 as covering of the mechanical oscillator 33 by the bulk material. This problem intensifies during applications in which the mechanical oscillator 33 must be operated with maximum sensitivity, i.e. that a switching operation is already triggered during a very small change in frequency and/or amplitude. This is the case for instance during the detection of very light substances, such as liquefied gases, for example.

By means of the present configuration, by measuring the environmental pressure, this shift of the resonance frequency f_(res) can be compensated. With a pressure range from −1 bar to 160 bar and a change of the resonance frequency of the mechanical oscillator 33×0.4 Hz/bar, this causes a frequency shift of 40 Hz during a change of the environmental pressure by 100 bar.

During level detecting in liquid gas, a frequency change of approximately 40 Hz is caused by covering the mechanical oscillator 33 with liquid gas. In this case, both a change of the environmental pressure by 100 bar as well as covering the mechanical oscillator 33 with bulk material would trigger a switch signal, although this is only desirable if the mechanical oscillator 33 is covered with the bulk material. This can be avoided by pressure compensation, as presently described.

Additionally, the resonance frequency f_(res) and the associated amplitude depends on the temperature θ of the measured medium. When the temperature θ increases, the density of the medium decreases during constant pressure and consequently the resonance frequency f_(res) of the mechanical oscillator increases.

In addition to the above pressure compensation, the present measuring instrument therefore preferably further comprises a temperature compensation. For this purpose, the measuring instrument is provided with a temperature sensor for detecting a process temperature inside of the container. The temperature sensor can be realized as a separate sensor and be disposed in the same process connection, or in one of the two existing sensors, for example integrated in the pressure sensor. As a result, this permits temperature compensation in addition to pressure compensation.

A temperature change would also be able to trigger such malfunction. Although temperature changes in the order of magnitude that alone would trigger an error rarely occur, it would nevertheless be sensible to provide temperature compensation for improving measuring accuracy, however.

Even during conditions of use when no switch state is caused by the potential pressure and/or temperature change, the measuring accuracy and switch reliability is significantly increased by the present configuration, so that advantages also exist here.

FIG. 2 illustrates an embodiment, in which the process connection 7 is formed as screw-in thread 72. In the embodiment illustrated in FIG. 2, the pressure sensor 5 is likewise disposed laterally to the level sensor 3 and is aligned in axial direction A. Both the level signal as well as the pressure signal are fed into a shared evaluation electronics 9, which, according to the above description, can also comprise a compensation module 91.

FIG. 3 illustrates an alternative form of design, wherein the process connection 7 in this embodiment is formed as second screw-in thread 73 with a reduced thread diameter in comparison with the embodiment according to FIG. 2. The second screw-in thread 73 has an extension piece 75, on which the pressure sensor 5 is disposed in radial alignment. As a result of such type of design, the surface of the process connection 7 required by the pressure sensor 5 is shifted from one front face of the screw-in thread 73 to a lateral area, so that a design with a correspondingly reduced diameter of the process connection 7 can be realized. Such designs are sensible in pump protection circuits, for example, because in this manner, a combination of tuning fork with integrated pressure measurement and a shared evaluation electronics 9 can protect a pump in a very compact form of design both against running dry as well as against excessive high pressure and excessive low pressure conditions.

The dependence of the resonance frequency f_(res) on the prevailing pressure p in the container, i.e. the process pressure, is shown graphically in FIG. 4a . The following table depicts the pressure dependency of the resonance frequency f_(res) at a temperature of 25° C.

P I bar 0 80 160 240 f_(res) I Hz 1373 1337 1306 1281

As can be seen in the above table and FIG. 4a , a linear relationship exists between pressure p and resonance frequency f_(res). Therefore, a simple compensation, for example by exact calculation or by means of a compensation table filed in a memory can proceed during otherwise constant environmental conditions.

In addition to the above-mentioned pressure dependency of the resonance frequency f_(res), a temperature dependence also exists. This is due to the fact that the density of a gas is also temperature-dependent. At a constant volume of the container and at constant pressure p, the density of the gas declines, so that the resonance frequency f_(res) increases. The above relationship between resonance frequency f_(res) and temperature θ for a constant volume and a pressure p of 160 bar is rendered in the following table.

υ/° C. −60 25 250 450 f_(res)/Hz 1271 1306 1335 1341

A graphic representation is shown in FIG. 4 b.

Depending on the field of applications of the measuring instrument 1 it may be sensible to provide only pressure compensation or additionally temperature compensation. As already previously mentioned, compensation is possible simply by a calculation or by means of a filed table.

LIST OF REFERENCE SYMBOLS

-   1 measuring instrument -   3 level sensor -   5 pressure sensor -   6 temperature sensor -   7 process connection -   9 evaluation electronics -   31 drive/detector -   33 mechanical oscillator -   35 first connecting line -   55 second connecting line -   71 flange -   72 first screw-in thread -   73 second screw-in thread -   75 extension piece -   91 compensation module -   A axial direction -   f_(res) resonance frequency -   p pressure -   θ temperature -   FIG. 4a : Y-axis: Resonance frequency/Hz; x-axis: Pressure/Bar -   FIG. 4b : Y-axis: Resonance frequency/Hz; x-axis: Temperature/° C. 

1.-18. (canceled)
 19. A measuring instrument, comprising: at least one level sensor for detecting a level of a medium having a level to be detected; said level sensor operably outputs a level signal at an output; at least one pressure sensor from which a pressure measurement signal is readable at said output; said level sensor and the pressure sensor having a shared process connection; and said measuring instrument configured in at least one of a group of configurations consisting of: (A) a first configuration wherein said process connection is embodied as one of a flange and a screw-in thread; and said pressure sensor is arranged axially relative to said measurement instrument, and (B) a second configuration wherein said process connection is embodied as said screw-in thread and further comprises an extension piece and said pressure sensor is arranged on said extension piece and further arranged laterally relative to said level sensor and aligned radially relative to said measuring instrument.
 20. The measuring instrument, according to claim 19, wherein: said level sensor is embodied as a vibration level sensor.
 21. The measuring instrument, according to claim 19 wherein: said pressure sensor is embodied as one of a capacitive and piezoresistive pressure sensor.
 22. The measuring instrument, according to claim 19, further comprising: a temperature sensor operable to output a temperature signal to said output for reading.
 23. The measuring instrument, according to claim 19, wherein: said level signal and the pressure signal are operably fed into a shared electronic evaluation unit (9).
 24. The measuring instrument, according to claim 22, wherein: said electronic evaluation unit further comprises: a compensation module which is operable to perform a pressure compensation of the level signal during an evaluation.
 25. The measuring instrument, according to claim 23, wherein: said level sensor is embodied as a vibration level sensor and the pressure compensation operably compensates at least one of a change in resonance frequency (fres) and an amplitude of a mechanical oscillator of the vibration level sensor that is due to pressure.
 26. The measuring instrument, according to claim 22, wherein: said temperature signal is fed into the shared electronic evaluation unit.
 27. The measuring instrument, according to claim 24, wherein: said compensation module is operable to conduct a temperature compensation of the level signal.
 28. The measuring instrument, according to claim 19, wherein: said measuring instrument is embodied as a flow limit switch.
 29. The measuring instrument, according to claim 19, wherein: said measuring instrument is arranged in a pump protection circuit.
 30. The measuring instrument, according to claim 19, wherein: said measuring instrument is embodied according to the EHEDG TYPE EL Class I standard.
 31. A method for operating a measuring instrument, comprising the steps of: providing at least one level sensor for detecting a level of a medium having a level to be detected; wherein: said level sensor operably outputs a level signal at an output; at least one pressure sensor from which a pressure measurement signal is readable at said output; said level sensor and the pressure sensor having a shared process connection; and said measuring instrument configured in at least one of a group of configurations consisting of: (C) a first configuration wherein said process connection is embodied as one of a flange and a screw-in thread; and said pressure sensor is arranged axially relative to said measurement instrument, and (D) a second configuration wherein said process connection is embodied as said screw-in thread and further comprises an extension piece and said pressure sensor is arranged on said extension piece and further arranged laterally relative to said level sensor and aligned radially relative to said measuring instrument; operating said level sensor and detecting a level signal; detecting a pressure signal; evaluating said signals output; and wherein said level sensor and the pressure sensor are operated at different times at least in sections.
 32. The method according to claim 31, wherein: said level sensor and said pressure sensor are operated cyclically alternately.
 33. The method according to claim 31, wherein: said the pressure sensor is operated continuously and that the level sensor is switched off cyclically during said continuous operation.
 34. The method according to claim 31, wherein: said level sensor outputs a trigger signal which marks the pressure signal as invalid during the operation of the level sensor.
 35. The method according to claim 31, further comprising the step of: detecting a temperature signal.
 36. The method according to claim 31, further comprising the step of: performing at least one of a pressure and a temperature compensation of the level signal. 